1. Field of the Invention
The present invention relates to flat panel display devices and, more particularly, to a method of driving a plasma display panel (PDP) among the flat panel display devices.
2. Discussion of Related Art
Generally, cells forming a pixel of PDP are discharging regions insulated by a spacer between upper layer and lower layer. These cells perform discharging by controlling voltages applied to horizontal and vertical electrodes formed on each of the upper and lower layers. An amount of discharged light is controlled according to the variation of the discharging time in the cell. The cells are arranged horizontally and vertically according to a specific size, forming the total field of PDP. The PDP displays digital video signals to a specific video field with a scanning driver and an addressing driver. That is, the addressing driver is connected to the vertical electrode, and the scanning driver is connected to the horizontal electrode. The former applies a write pulse for inputting the digital video signal and an erase pulse for stopping the cell discharged by the video signal. The latter realizes a matrix type field by applying the scan pulse for causing discharge according to the video signal and the sustain pulse for sustaining the discharge caused the scan pulse for a predetermined time to the cell on the corresponding horizontal line. To make a continuous image, the following operations are repeated: the scan and write pulses are applied to PDP at the same time in order to discharge; the field is sustained at the discharged state for a predetermined time by the sustain pulse; the field is erased by the erase pulse to display the next field. That is, the preceding discharging, sustaining and erasing operations are repeated to display the next field.
As described above, eight sub-fields are overlapped and sequentially displayed as one field by controlling the scanning and addressing drivers. The method is called sub-field driving method.
In this sub-field driving method, eight sub-fields should be sequentially concentrated to make one image. An one-bit digital image signal corresponds to each of the cells and is applied to each of the 960 lines, thereby forming one PDP sub-field having equal brightness. Therefore, when the eight sub-fields each having different brightness are collected by the digital video signals of eight bits, they make one image. Theses images continuously arranged form the moving picture. That is, the eight sub-fields are classified into sub-fields having different brightness. A first sub-field is formed of digital video signal of most significant bit (MSB), which has highest brightness among those digital signals of eight bits. Each of second through seventh sub-fields has the differentially lower brightness than the first sub-field. An eighth sub-field is made of the digital signal of least significant bits (LSB).
A method of operating these sub-fields is to display eight sub-fields having the digital signals of eight bits from first LSB to last LSB. In other words, those eight sub-fields are formed by scanning the first sub-field for the discharging time T, and another second through eight sub-fields for T/2, T/4, T/8, T/16, T/32, T/64 and T/128. In this way, a complete video field is displayed by a persistence of vision with respect to light emitted from each of the sub-field. To form the sub-field, a predetermined period of time is needed to scan all of the horizontal electrodes. Each of the cells can maintain discharging only during the predetermined period of time, that is, the average time exclusive of scanning time, the average time being allocated to each sub-field. The scanning time increases in proportion to the number of the horizontal electrodes. As one cannot maintain the discharging during the scanning time, contrast and brightness of PDP may be decreased, so that the scanning time should be reduced.
In addition, as the difference of discharging time between the upper bits and the lower bits in forming the sub-field and the sub-fields are formed sequentially, a flickering occurs due to the difference of discharging time. To reduce the flickering, it is needed to arrange the sub-field of upper bit requiring the long discharging time and the other sub-field of lower bit requiring the short discharging time in the appropriate order.
In the sub-field operating method, a gradual gray scale required for displaying image realizes the length of time in which each cell is discharged and maintained differently within the predetermined period of time (1/30 sec. in case of NTSC TV) given to display total image. Here, the brightness of the field is determined by the gray scale when each cell is operated maximally. To increase this brightness, the operating circuit should be designed to maximally maintain the cell discharging time with the time given for structuring one field. The contrast, the difference in tone between dark area and bright area, can be controlled by the brightness determined. To increase the contrast, it is needed to darken the background and increase the brightness. Specifically, in case of flat display panel devices for high resolution television, as the gray scale should be 256 units, the resolution should be 1280.times.1024 and contrast should be more than 100:1 under the light of 200 Lux, 8 bits of R. G. B. data are respectively required for the video digital signals for displaying the gray scale of 256 units, and the discharging time of the cell should be maximally maintained to gain the required brightness and contrast.
An conventional PDP operating method will be described below.
FIG. 1 is a sectional view of an surface discharged cell of AC PDP having the three electrodes typically used.
A spacer 10 maintains first and second insulting layers 1 and 2, and insulates the gap among cells. Row electrodes are made of a scan electrode 11 and a common electrode 12 and arranged on the first insulating layer 1 in parallel.
The column electrode 4 is arranged on the second insulating substrate 2 to oppose to the row electrode, keeping a matrix shape. The first and second insulating layers 5 and 6 respectively covers the row electrode and the column electrode 4, protecting the electrodes. As the electrodes are covered with the insulating layers, if a series voltage is applied between the electrodes to discharge, the discharging is erased instantly. In case of the PDP having the thus-structured electrodes, a parallel voltage whose polarity is continuously inverted should be applied between electrodes to maintain the discharging.
A passivation layer 7 is deposited on the second insulating layer 5. The passivation layer 7 is typically made of thin film of MgO in order to protect the insulating layer 5, enlarge its lifetime, increase the efficiency of emitting second electrons and reduce the variation in the discharging characteristics caused by the contamination of oxide in refractory metals.
A fluorescent layer 9 is deposited on the second insulating layer 2 including the spacer 10, and excited by ultraviolet rays generated due to the discharging, generating visible rays of R. G. B. colors. A discharge space 8 is the cell space in which the discharging is performed. To increase the ultraviolet ray emitting efficiency, a mixture of Ar and Xe is filled in the space.
FIG. 2 shows an arrangement of the conventional AC PDP electrodes.
Each of the cells 13 is formed at the intersecting point where the column electrode intersects with the row electrode. The row electrode has scan electrode group S.sub.1 through S.sub.m mainly used for scanning field, and common electrode group C.sub.1 through C.sub.m mainly used for maintaining the discharging. The group of the row electrode is made of the address electrodes D.sub.1 through D.sub.n used for data input.
A sealing region 14 is used for maintaining vacuum state of the entire PDP, and formed by inserting the spacer 10 between the first and second insulating layers 1 and 2 and sealing the edge of the PDP with adhesives.
An operation waveform with respect to each electrode and its sub-field scanning method are classified into two method. Firstly, their operation waveforms are shown in FIG. 3. A sustain pulse is applied to the common electrodes C.sub.1 to C.sub.m to maintain the discharging of the cell 13. The scan electrodes S.sub.1 to S.sub.m receive another sustain pulse whose shape is the same as the pulse of the common electrodes C.sub.1 to C.sub.m but its position of the time difference is different. If one of the cells S.sub.1 and D.sub.1 should be discharged, a positive data pulse is input to the address electrode D.sub.1 and the scan pulse is synchronized with the data pulse and thus input to the scan electrode S.sub.1, the voltage level between the electrodes S.sub.1 and D.sub.1 is increased higher than the threshold level and discharged.
This state is maintained until the next erase pulse is input thereto by the electronic field generated with the discharged particles in the insulating layer and the electronic field generated by the sustain pulse of the scan and common electrodes S.sub.1 and C.sub.1. If an erase pulse having lower amplitude than the scan pulse is applied thereto, the sum of the electronic field of discharged particles and another electronic field of the erase pulse is too small to continuously maintain the discharging, so that the discharging is erased when the next sustain pulse is applied thereto.
In brief, the scan electrodes perform the sustain function and scanning function but the common electrodes only perform the sustain function. The address electrodes input data for structuring field.
Each pulse through the scan electrodes S.sub.1 to S.sub.m performs the sequential scanning operation to Sn for 2 to 3 .mu.s. In synchronization with the scanning operation, if data pulses as many as the number of the lines in the scan electrodes are applied through the address electrodes D.sub.1 to D.sub.n, each cell of the PDP field momentously display the sub-field of the equal brightness to the random digital image data.
A method of displaying eight sub-fields corresponding to each of the 8 bits digital image data is described below.
FIG. 4 shows a scan method in the conventional sub-field operation method for realizing the gray scale of 256 units on basis of the operation waveforms of FIG. 3. Here, a field is made of eight sub-fields, and the time allocated to each of the sub-fields is constant as T.sub.A. Accordingly, as a field is made of eight sub-fields, the required time T.sub.FIELD is 8T.sub.A. Here, within T.sub.A, times of T.sub.A /2, T.sub.A /4, T.sub.A /8, T.sub.A /16, T.sub.A /32, T.sub.A /64, and T.sub.A /128 are only used for discharging sequentially from MSB to LSB. Therefore, within 8T.sub.A for structuring a field, the time T.sub.s for discharging is 2T.sub.A, and the time T.sub.NS not allowed to discharge is 6T.sub.A. Waste and efficiency of the wasted time T.sub.NS is as follows. ##EQU1##
The above equation 1 shows that the time which can be used actually for discharging is below 25% in case of using PDP to which the sub-field operating method is applied, and it means that this plays a main factor of decreasing brightness in the PDP.
Its operational waveform and scan method are respectively shown in FIGS. 5 and 6.
In the waveforms of FIG. 5, the first sub-field is divided into a reset period, an address period, and a sustain period, completely separating addressing from sustaining. First, within the reset period, at point (a), the section of the scan electrode from S.sub.1 to S.sub.m has the value of 0(V) and a light pulse is input to the common electrodes S.sub.1 to S.sub.m, so that an initial discharging starts between all common electrodes and scan electrodes.
Thereafter, at point (b), a sustain pulse is input to each of the scan electrodes and therefore the sustain discharging is generated. At point (c), the total erase pulse is input to the common electrodes, erasing the discharging of all cells. The next address period starts with point (d) and ends right before the point (f). Here, at the point (d), the positive (+) voltage of the same value is applied to each of the scan and common electrodes in order to prepare for the addressing starting from point (e). At the point (e), the data pulse of the address electrode is synchronized with the scan pulse of the scan electrode and input. If the positive data pulses are input to the cells belonging to the row electrodes to which the scan pulse is input, the wall electrons are formed due to discharging, generating the sustain discharging in the following sustain period. If the data pulse of 0(V) is input, the sustain discharging cannot be generated in the sustain period. The scan pulses are sequentially input to all row electrodes in the order of S.sub.1, S.sub.2, S.sub.3 to S.sub.m.
At last, the sustain period starts with the point (f). The method of inputting pulses to the scan and common electrodes is the same as the sustain pulse inputting method of FIG. 3.
FIG. 6 shows the conventional sub-field scan method for realizing gray scale of 256 units on basis of the operational waveforms of FIG. 5.
A field of PDP is made of eight sub-fields, SF1 to SF8. Each of sub-fields is made of reset, address and sustain periods as of FIG. 5. The reset period is positioned at the start point of all sub-fields and renders the initial state where the addressing is available by erasing the discharging of all cells. The time allocated to the reset period is constant to all sub-fields.
The sustain period maintains continuously the discharging of the cells where write discharging is generated in the address period. If the sustain period of SF1 is set to T, the sustain periods of SF2, SF3, SF4, SF5, SF6, SF7 through SF8 are made to 2T, 4T, 8T, 16T, 32T, 64T and 128T, thereby realizing them as the gray scale of 256 units.
In the address period, to enable the cells, which should maintain discharging during the sustain period, to write-discharge, the PDP is scanned sequentially from the highest line by one scan electrode of a row, and the time allocated to each address period is constant with respect to all sub-fields.
But, the time allocated to the address period increases in proportion to the increase in the number of the row electrodes to scan and the time allocated to the sustain period decreases. Therefore, its light emitting efficiency is decreased and it is hard to realize the gray scale of 256 units in the high resolution PDP.
Consequently, the conventional art displays eight sub-fields corresponding to the digital image data of eight bits to realize one field. But, its discharging time in the address period is 25% of the time for displaying one field, having low efficiency, the brightness off PDP is drastically decreased. Specifically, the conventional art has the problem in realizing the gray scale of 256 units in the PDP having high resolution above 640.times.480 which can be made by keeping the scanning time maximally.